AC power converter

ABSTRACT

The present invention relates to apparatus used in power control, such as apparatus for controlling the power level in an AC electrical circuit. Such an apparatus is referred to as an AC power converter. In one form, the present invention particularly relates to a power converter suitable for use in lighting dimmer control although other applications of the present invention are not to be excluded from the scope of the present application. A number of aspects of the power converter are disclosed.  
     One aspect is directed to the problem of freewheeling in a power converter. In another aspect the linearity of the circuit transfer characteristic is improved when operating in response to small load outputs. A further aspect relates to surge detection by controlling the pulse width and accordingly, the amplitude of a spike. In a still further aspect, communication between a number of power converters uses a network such as ethernet, or preferably LON™.

FIELD

[0001] The present invention relates to apparatus used in power control,such as apparatus for controlling the power level in an AC electricalcircuit. Such an apparatus is referred to as an AC power converter. Inone form, the present invention particularly relates to a powerconverter suitable for use in lighting dimmer control although otherapplications of the present invention are not to be excluded from thescope of the present application. A number of aspects of the powerconverter are disclosed.

BACKGROUND OF THE INVENTION

[0002] The prior art teaches us about power converters based on avariety of technologies. These techniques can be broadly divided intolinear and switching types, and U.S. Pat. No. 5,500,575 discloses anumber of examples of linear and switching power converter technologies.Two of these are “Phase Control” and PWM.

[0003] One converter disclosed in the prior art is a high frequency,switch-mode power converter operating on the Pulse Width Modulation or“PWM” principal. PWM converters may again be further subdivided into avariety of types: “Direct” converters, “Indirect” converters, “L-bridge”and “H-bridge” for example.

[0004] A number of problems exist with prior art PWM power converters.

[0005] For a PWM converter of the prior art, in normal operation, poweris transferred from the mains to the load when the input voltage andcurrent waveforms are substantially of the same polarity and when themain switch is “on”. This type of converter includes an output filterwithout which the converter output to the load would contain substantialharmonics of the switching frequency.

[0006] One problem of the prior art occurs when the load is reactive(Capacitive or Inductive). Referring to FIGS. 1A, 1B, 1C and 1D, aschematic of prior art power converter 1 is shown having an input 2comprising active terminal 3 and neutral terminal 4, input filter 5,main-switch 6, output filter 7 and load 8. In the positive half cycle ofthe AC input waveform, current flows as shown by arrow 9. Conversely, inthe negative half cycle, current flows as shown by arrow 10.

[0007] Consider the inductive load case and when the main-switch isturned “off”. Due to the residual energy stored in the inductor 8, thecurrent flowing in the load has a tendency to continue to flow even whenthe main-switch is “off”. However, because there is no path for thecurrent flow, the energy stored in the inductor causes a voltage todevelop across the load. Referring to FIG. 1B, the arrow 11 indicatesthis voltage.

[0008] However, if this voltage is allowed to develop unchecked, it canrise to levels that can damage the components of the power converter: avoltage “spike”. One way to avoid this problem is to provide a path forthe inductor current to continue to flow even when the main-switch is“off”. This current is often termed “free-wheeling current”. Typically asecondary switch device 12 (FIG. 1C), also known as sub-switches isprovided to carry this freewheeling current 13 and thus eliminate thevoltage spike. Typically the sub-switches are arranged in pairs, oneeach for positive and negative half cycles however, for simplicity, FIG.1C shows only one such switch. The sub-switches of the prior art aretypically operated at line frequency and when the load current polarityand input voltage polarity are the same, the prior art workssatisfactorily.

[0009] However, when the load current and input voltage are of oppositepolarity, as illustrated in FIG. 1D, the prior art no longer workssatisfactorily. In the figure it can be seen that the “on” main-switchand “on” sub-switch form a short circuit across the AC Input 2. In thiscircumstance special techniques are required to prevent this shortcircuit and consequent dangerous current spike.

[0010] These techniques are typically complex, bulky, inefficient,expensive or only partially successful. Furthermore, the output filterrenders almost all loads to be reactive and thus exposed to thisproblem.

[0011] Canadian Patent 2107490 discloses one arrangement designed toaddress the problem of freewheeling currents. This arrangementnecessitates the use of sub-switches which are controlled to switch onwhen the Main switch is switched off. In this way, the sub-switchesprovide the current path for the freewheeling current. One problem,however, with this technique is that the sub-switches must be of asimilar specification to Main switch because they usually operate at thesame frequency as the Main switch. This leads to relatively higher costand lower efficiency.

[0012] In the prior art, another separate and distinct problem exists,namely that prior art circuits are usually designed to operate optimallyat a particular load output. However, problems occur when the circuit isused at a relatively lower power output than that of the initial design.

[0013] The output filter as illustrated schematically in FIG. 1A usuallycomprises an inductor and capacitor. Under normal operation with largeloads with low internal impedance, the charge that would otherwiseaccumulate on the filter's capacitor is discharged into the load.However, with small loads, the charge tends to accumulate on thecapacitor with each PWM switching cycle. This is because the chargingsource impedance (the filter) is higher than the discharge impedance(the load) so that the capacitor voltage tends to approach linepotential with each successive PWM pulse. In effect, when themain-switch is “off”, the load continues to be driven by the capacitor.The result is that with small loads and low to medium output levelsettings, the output of the converter is distorted and the transfercharacteristic of the converter is impaired. FIG. 2 illustrates thetransfer characteristic for a power converter designed for an output ofup to 3KW. On the vertical axis, output voltage is illustrated, and inthis example from 0 to 250 volts. It would be readily appreciated thatthe range of voltage is not limiting in describing the presentinvention. On the horizontal axis, a percentage of pulse width of PWM isillustrated, ranging from 0 to 255, being an eight-bit binaryrepresentation of 0 to 100%. As can be seen by the line denoted 13, thecharacteristic is relatively linear for operation at 3KW output. This isthe intended transfer characteristic for the particular circuit plotted.Compare this, however, to line 14 illustrating operation at 25W output,line 15 illustrating operation at 200W output and line 16 illustratingoperation at 650W output. The transfer characteristic as represented byeach of numerals 14, 15 and 16 is not relatively linear. Thus, theoutput of the circuit is not linearly proportional to the percentagePWM, resulting in the output for line 14 (at 50 PWM) being approximately160 volts rather than approximately 40 volts for line 13.

[0014] U.S. Pat. No. 5,500,575 describes a means of using thesub-switches to discharge the filter capacitor under certain loadconditions. The problem with this technique is that it is not consideredprogressive in operation, it is relatively complex to implement andrequires high-speed sub-switches.

[0015] Still further problems are associated with the prior art inrelation to detecting and minimising the problems resultant from surgecurrents circulating within the circuit. U.S. Pat. No. 5,500,575discloses a form of current limiting however the current sensing is doneat the load side of the output filter and is thus considered to be notas effective because it is affected by the filter time constant. Also,the prior art cannot protect the circuit, particularly the main switch,against over-current and/or short circuit in the sub-switches.

[0016] Another problem with the prior art concerns remote control.

[0017] Theatrical/Professional dimmers have been remotely controlled formany years, even in times preceding solid state phase control dimmers.In this context “Remote Control” refers to the ability to command theoutput level of the dimmer from a remote location. This has beenaccomplished in a number of ways, ranging from individual control wiresfor each channel carrying a voltage reference proportional to level(e.g. 0 to 10 Volts), to various analogue and digital multiplexingschemes. AMX and DMX are acronyms for Analogue and Digital Multiplexcontrol standards.

[0018] Around 1987 when the first “Digital” dimmers were introduced,this remote control concept began to evolve to encompass a variety offunctions in addition to simply commanding output level. Variousproprietary protocols have emerged as a result.

[0019] Another byproduct of the emergence of the Digital Dimmer was theprovision of a wide range of functions that could be performed withinthe dimmer. Previously, about the only thing that the user could performin relation to the dimmer was to modify its calibration with a trimpotof something similar. Now, however, it is common for a dimmer to possesscomplex user interface, sometimes graphical, via which a huge number ofparameters can be altered. Many current generation products can evenfunction usefully without any “upstream” controller. This interfacewould typically comprise a display means (eg. LCD) and a number of dialsand/or push buttons. Typically, this user interface is interfaced to theelectronics of the dimmer pack in a manner that does not readily adaptto a remote or networked location of this interface.

[0020] A number of manufacturers have developed remote control andnetworking systems and protocols which enable many of these features tobe accessed from a remote location. However these systems do notreplicate the entirety of the local user interface and nor do theyemploy the same graphical and/or physical user interface means.

[0021] The current technology has a number of problems.

[0022] One problem is that the provision of the user interface necessaryfor local access to the complex internal function of the dimmer addscost, complexity and unnecessary redundancy to the dimmer system. It isusual to employ theatrical dimmers in large numbers of channels. Atypical theatre or concert system may use several hundred channels.Typically, the channels are arranged into “units” which might contain 12channels each. In the existing technology, each unit includes a userinterface facility. FIG. 11 illustrates such a collection of dimmerunits by reference to numeral 59, called a “rack”.

[0023] Another problem with the prior art is that the proliferation ofavailable functions available via the typical front panel user interfacerenders the dimmer complicated to use and requires a heightened degreeof expertise on the part of the user.

[0024] Another problem associated with the prior art is that providingready access to the complex internal functions of the dimmer means thatit is possible for unauthorised or accidental changes to be made to thesettings of the dimmer.

SUMMARY OF INVENTION

[0025] The present invention seeks to alleviate at least one prior artproblem.

Main Switch

[0026] In one aspect of the present invention a power converter isprovided that can adapt to a wide range of load reactances, whethercapacitive, inductive and/or resistive loads.

[0027] The present invention provides a power converter including aninput means for receiving supply power, a switch means responsive tocontrol means for providing preliminary control of power delivered to anoutput load, and a detecting means to detect a difference in polarity oramplitude between selected waveforms or points in the converter. In thepresent invention, the control means controls the operation of theswitch means in a manner that enables the switch means to be ‘on’ whenrequired for control purposes. Preferably, the switch means is enabled‘on’ when there is a difference in polarity detected. Furthermore,preferably the switch means is enabled ‘on’ when a difference inpolarity between voltage and current waveforms is detected.

[0028] This is basically accomplished by controlling the mainswitch/driver of the converter in a manner that keeps the main switch“on” when there is a difference in polarity detected between the inputvoltage and current to the converter or when a difference in amplitudeis detected between the voltage and current waveforms flowing throughthe converter. In this way the energy stored in the reactive load whichmight otherwise give rise to destructive spikes (current and/or voltage)in the converter is directed back to the mains supply. Preferably, adifference in polarity may reside between input voltage and voltageacross the main switch, or between voltage and current waveforms throughthe main switch.

[0029] The present invention will be described as it would be applied toan L-Bridge, Direct conversion PWM power converter of basic circuittopology similar to that described in U.S. Pat. No. 5,424,618. This typeof converter comprises a “main switch” which, essentially, connects themains supply to the load, and a “sub switch” which is, essentially,connected in parallel with the load. As previously described, PWMconverters may be subdivided into a variety of types: “Direct”converters, “Indirect” converters, “L-bridge” and “H-bridge” forexample. Although the present invention is disclosed with reference to aL-Bridge Direct-converter design, it should be noted that some aspectsof the present invention have equal application to other types ofconverters. The embodiment described is a preferred embodiment, but notthe only embodiment.

[0030] One application of the present invention is directed to a dimmerin which pulse width modulation (PWM) of the input waveform is used tocontrol the output waveform. In a preferred form, a high frequency PWMsignal samples the input waveform.

[0031] The pulse width modulation may be implemented in conjunction withan IGBT or other switching device (eg. MOSFET) and control circuit usedas a “main switch”. By changing the PWM duty cycle, it is possible toeffect an amplitude change in the output waveform.

[0032] If there is an inductive or capacitive load, there is a relativelag or lead with regard to the voltage and current waveforms of thepower signal. In use, when the control circuit of the present inventiondetects a lead or lag, that is a phase or polarity difference betweenthe voltage and current waveforms, as noted above, the main switch isturned “on” and remains “on” whilst the polarity of the voltage andcurrent waveforms is opposed. Thus, the pulse width modulation isaltered. Having the main switch “on”, enables energy, which in the priorart would be left to freewheel, to be fed back to the input supply. Thisreduces dissipation and enhances reliability and efficiency.

[0033] Where a difference in amplitude is used as a basis for detection,an appropriate amplitude detector can be used. One example may be a zerocrossing detector, used to detect whether the voltage and currentwaveforms both cross ‘zero’ at the same time. If not, in accordance withthe present invention, the main switch can be turned ‘on’ (byappropriate logic) until it has been detected that both waveforms havecrossed ‘zero’.

Transfer Characteristic

[0034] In accordance with another aspect of the present invention, theproblem of charge build up on the output filter capacitor is addressedin a manner that substantially improves the linearity of the circuittransfer characteristic when operating in response to small loadoutputs.

[0035] In this regard, the present invention provides an adaptiveinductance for use in an output filter of the converter. The inductanceis designed to be adaptive to the current flowing in the load.

[0036] One embodiment includes the adaptive inductance of the presentinvention in a power converter with an “LC” output filter of “L” sectiontopology in which the inductor is adaptive to current in the mannerdescribed above. In one particular embodiment, the inductor wouldcomprise two otherwise conventional inductors arranged in series.

[0037] In the described embodiment, the first inductance has impedancewhich is relatively low at line frequency and which is relatively highat switching frequency, when compared to the load impedance. The secondinductance has relatively high impedance when the load current isrelatively low and has relatively low impedance at relatively high loadcurrents at all frequencies.

[0038] The present invention is based on the realisation that theproblem of non-linear transfer characteristic performance at relativelylow power output is due to the design of output filters used prior tothis invention. Typically the output filter, as coupled to the load, isof “L” section “LC” topology, that is to say that, it is composed ofseries inductance and parallel capacitance. In the prior art, theinductance of the output filter is specified to present a high impedanceto switching harmonics whilst a relatively low impedance to linefrequency. Also, in the prior art, the capacitance of the output filteris designed to present low impedance at switching frequencies and highimpedance at line frequencies. Together the inductance and capacitanceeffectively block the passage of switching frequencies and harmonics tothe load.

[0039] These filters work satisfactorily when the load impedance is low,or comparable to the inductor impedance at switching frequencies. Innormal operation, with each switching cycle (PWM cycle), charge isdeposited on the capacitor of the filter via the inductor such that thecapacitor voltage tends toward line voltage. When the main-switch turns“off”, the capacitor charge is dissipated into the load and thecapacitor voltage tends toward zero. This is repeated at high frequencyfor each switching cycle. Ideally, the average voltage across thecapacitor would be a fraction of the line voltage proportional to thePWM proportion.

[0040] However, the present invention is focussed on solving, at least,the problem of when the load impedance is high, the capacitor chargecannot dissipate adequately during the main-switch “off” state andcharge (voltage) tends to accumulate on the capacitor.

[0041] In the present invention, by providing a second inductance inseries (whether the second inductance is provided as a separate elementor incorporated into existing circuit elements, such as the existingcircuit inductance L) with the existing output filter inductance whichis designed to present high impedance at switching frequencies atrelatively low currents (which corresponds to low load conditions) ithas been found that the switching currents flowing in the filter, at lowloads, are reduced so that the load impedance is sufficient to preventthe build-up of charge on the output capacitor. The second inductance isdesigned so that when the converter load is increased, the secondinductor core progressively saturates so that it presents low impedanceto switching frequencies at high load conditions. By carefully selectingthe value and saturation characteristics of the second inductance andits core material and its characteristics, including permeability, it ispossible to alleviate the problem of the charge build-up problem in theoutput filter typical with that of the prior art output filter designs.

[0042] The preceding example uses two series inductors to achieve therequired characteristic, which is adaptive to load current. Thoseskilled in the art would appreciate that the required inductancecharacteristic might be achieved via other means.

[0043] For example, the inductance might be constructed using multiplecore elements shared by one or more windings. In this case the corewould be selected, by virtue of material and/or design, so that the netpermeability of the core, and therefore, the inductance possessed acharacteristic generally inversely proportional to current.

[0044] Alternatively, a single core might be employed, that core beingcomposed of material, perhaps composite, such that the requiredpermeability, and thus inductance, characteristic is obtained.

[0045] Essentially, the inductance characteristic should be designed, asnear as possible, so that the charge and discharge time constants, withrespect to switching PWM) frequencies, for the converter output filtercapacitor tend to be equal for all load impedances.

Surge Detection

[0046] The present invention also serves to detect and limit surgecurrents and therefore reduces electrical and thermal stresses applyingto circuit components of the power converter and externally connectedequipment including the load.

[0047] If a current spike is detected, the pulse width is decreased (PWMturned off) and accordingly, the amplitude of the spike is reduced, thusovercoming or controlling the output of the spike.

[0048] The present invention provides a method of controllingover-current and/or short circuit conditions in a circuit by providingPWM sampling of the input waveform, measuring current as it passesthrough a mainswitch, turning the mainswitch off in response to thecurrent measurement of an over-current condition and adjusting the PWMin an over-current condition, and at a frequency that serves to rapidlyattenuate the current through the converter.

[0049] The essence of this aspect of the invention is that the currentmeasurement is made as it passes through the main-switch as opposed tomeasuring the load current. This means that the current measurement is“real-time” and can be used to control the main-switch current at thefrequency of the PWM. The prior art would teach us to measure the output(load) current and to use this measurement to modify the PWM. This maybe too slow (due to the effect of the output filter) to adequatelyprotect the power transistor of the main-switch. Furthermore, the priorart cannot measure the current flowing in the sub-switches of theconverter and therefore cannot protect them or the main-switch in somecircumstances. The present aspect enables more accurate surge currentlimiting in the main-switch, enhancing the reliability of the converter.

Networking

[0050] The current invention addresses the problems noted above byinterfacing the local user interface device(s) to the dimmer using alocal area network and making the user interface panel a detachableplug-in to the dimmer unit. In fact, it need never be attached at all.In the present aspect, implementation of this concept, a vestigialcontrol panel is provided in addition to the networked (main) panel toenable basic functionality when the main panel is removed. This wouldnot be a mandatory requirement.

[0051] In this regard, the present invention provides a control unitadapted to control a power converter, the control unit being providedintegrally with the power converter in which the control unit isattached to and communicates directly to the power converter, orremotely of the power converter in which the control unit is detachedand communicates remotely via a suitable mode of communication to thepower converter. A network of power converters coupled via acommunication network and including this control unit is also provided.

[0052] This means that, say, one user interface module may be used tocontrol a multiplicity of dimmer units or racks. This local controlmodule may be located either nearby or remotely. This remote controlnetwork may be independent from the lighting control network.

[0053] It also means that the user interface may be removed entirely(since it is not essential for the basic function of the dimmer) thusadding security from tampering.

[0054] Other aspects of invention are also disclosed.

[0055] A preferred embodiment relating to aspects of the presentinvention will now be described with reference to the accompanyingdrawings, in which:

[0056]FIGS. 1A, 1B, 1C and 1D illustrate problems associated with priorart power converters,

[0057]FIG. 2 illustrates transfer characteristics of a prior art powerconverter,

[0058]FIG. 3 illustrates in block diagram form one embodiment an ACPower Converter according to the present invention,

[0059]FIG. 4 provides more detail of an embodiment of an AC PowerConverter according to the present invention,

[0060]FIGS. 5A, 5B and 5C illustrate a number of alternative switcharrangements which may be used in place of the main switch and/or thesubswitch(s) to suit different applications,

[0061]FIGS. 6A, 6B, 6C and 6D illustrate waveforms associated with thepresent invention,

[0062]FIGS. 7 and 8 illustrate waveforms (Inductive Load Current Flow)and (Capacitive Load Current Flow) respectively related current flowsthrough an embodiment of the present invention,

[0063]FIG. 9 illustrates a control circuit block diagram,

[0064]FIG. 10 illustrates transfer characteristic or a power converteraccording to the present invention,

[0065]FIG. 11 illustrates a networking aspect of the present invention,and

[0066]FIG. 12 illustrates one embodiment of an adaptive inductance.

GENERAL DESCRIPTION

[0067] One embodiment of the present invention is a microprocessor basedsine wave AC power converter, designed for adapting to various loadingconditions (resistive, capacitive and inductive loads). Themicroprocessor may also be provided in the form of control logic, havinga suitable/required controlling function. The present invention alsoaccommodates loads of varying reactance such as a discharge lamp, whichin operation displays a change in reactance value as it heats up. Thepresent invention can be accommodating of many and different types ofloads because the invention monitors the polarity difference.

[0068] Furthermore, in the present invention, the power factor remainssubstantially constant while dimming occurs, and does not worsensignificantly at lower power (greater dimming) levels. Also detection ofover current and/or short circuit current conditions can reduce oreliminate the need for Circuit Breakers (CB). Still further, the inputand output of the present invention share the same neutral line enablingthe load energy to feed back through the mains supply. This allows theuse of standard cabling and eliminates any need for oversized orseparated neutral wiring. Other advantages of the present invention willalso be described.

Function

[0069] Generate a variable AC voltage output with the same frequency andform as the input

[0070] Adaptive load control for various types of load (reactive andresistive)

[0071] Over-load and short circuit discriminating

[0072] Over-current presetting and (PWM) cycle-by-cycle shutdown ondetecting over-current

[0073] shutdown output on detecting short-circuit current

[0074] Other panel display functions etc.

Circuit Structure

[0075] The main structure of one embodiment of an AC Power Converteraccording to the present invention is illustrated schematically in FIG.3 and is shown in FIG. 4 in more detail. The same numeral is used todenote the same element, even if shown in different figures.

[0076] There are nine parts usually included in an AC Power Converterbut which are not all necessarily essential to the present invention.These are:

[0077] 1. Iput Filter

[0078] Input filter block 5 is composed of an inductor (L1) and acapacitor (C1), it is a simple LC low pass filter. The main purpose ofthe input filter is to block high frequency harmonics from feeding backthrough the main network from the AC Power converter.

[0079] 2. Mian Switch (Driver & Protection)

[0080] Main switch block 6 is composed of an IGBT, MOSFET or similar,power switch (Q1), a bridge rectifier (D1, but shown as a diode bridgeD1/1, D1/2, D1/3 and D1/4) and a control circuit including over-voltageprotection 17 and current limiting and power switch driver circuit 18.Control of the main switch on and off will change the amplitude of theoutput voltage for adapting different loading condition. Any one of theswitch arrangements as illustrated in FIG. 4, 5A, 5B or 5C may be usedas the ‘main switch’. Equally other switch arrangement(s) may also beused in providing a switching function.

[0081] 3. Sub Switches (Driver & Protection)

[0082] Sub switches block 12 is composed of two IGBTs, MOSFETs orsimilar. (Q2, Q3), two ultra-fast soft-recovery diodes (D2, D3) and twosub switch driver circuits 20 and 21 controlling each sub switch ‘on’and ‘off’ to control the freewheel current. Over-voltage protection 22and 23 is also preferably provided. Equally, any one of the switcharrangements as illustrated in FIG. 4, 5A, 5B or 5C may be used as the‘sub switch’ and/or other switch arrangement(s) may also be used inproviding a sub switching function.

[0083] 4. Output Filter

[0084] Output filter block 7 is composed on an inductor (L2) and acapacitor (C2), it is a simple LC low pass filter. The main purpose ofthe output filter is to filter out high frequency harmonics to the load.In accordance with a further and distinct aspect of the presentinvention, a further inductor L3 is provided in series with inductor L2to provide a relatively linear transfer characteristic throughout theoutput range of the circuit.

[0085] 5. Input Voltage Polarity Sensing Circuit

[0086] Input voltage polarity sensing circuit 24 is opto-isolated fromthe main switch control circuit, and preferably provides an inputvoltage polarity signal to the main switch control circuit which alongwith the load current direction signal is used to drive the main switchand sub switches in the correct sequence.

[0087] 6. Current Direction Sensing Circuit

[0088] Current direction sensing circuit 25 is opto-isolated from themain switch control circuit, and preferably provides a load currentdirection signal to the main switch control circuit which along with theinput voltage polarity signal is used to drive main switch and subswitches in the correct sequence.

[0089] 7. Output Voltage Amplitude Sensing Circuit

[0090] Output voltage amplitude sensing circuit 26 is also opto-isolatedfrom the central control circuit, and preferably provides an analogsignal which is proportional to the output voltage to the centralcontrol circuit. This signal can be monitored by the control logic,enabling it to regulate the output voltage. Circuit 26 can also be usedto detect a short circuit.

[0091] 8. Temperature Sensing Circuit

[0092] Temperature sensing circuit 27 provides an analog signal,proportional to heatsink or other ambient temperature, to the centralcontrol circuit. This signal is monitored by the control logic, forexample enabling it to turn the main switch off when the temperature isover a certain degree (say 90° C.) or enabling reduction of the poweroutput.

[0093] 9. Central Control Circuit

[0094] The central control circuit 19 comprises the requiredmicroprocessor(s) and/or associated software and/or hardware circuits.It is preferred that switch control and other important signals arehandled by discrete hardware for a faster response. The PWM frequencybase can be generated by appropriate circuitry which can also processother less time critical system states signals (like over-voltage,temperature, input settings, etc).

Main Switch Principle

[0095] A load may be considered as one of three types: Capacitive loadinput current leads input voltage Inductive load input current lagsinput voltage Resistive load input current and input voltage are inphase

[0096] Basically, one aspect of the invention operates as follows:

[0097] The IGBT and associated controlling circuit form the “mainswitch” 6. By changing the duty cycle of the PWM, (PWM (1)), theamplitude of the waveform is affected. Looking at FIG. 6a, waveforms 28and 29 for voltage (V) and current (I) respectively are shown out ofphase, as would be the case for a inductive load. FIG. 6b shows waveform30 the PWM(1), which when applied to the waveform of FIG. 6a, results inthe waveform 31 of reduced amplitude (not changed frequency) of FIG. 6c.

[0098] One aspect of the present invention is to keep ‘on’ the mainswitch when the is a polarity difference between voltage and currentwaveforms 28, 29. FIG. 6d illustrates this in waveform 32, and theextended ‘on’ time illustrated at 33. The PWM(2) is altered to keep themain switch “on” for at least the period during which the V & Iwaveforms are of opposing polarity. This “on” time enables energy storedin the load (8 of FIG. 3), to be feedback to the supply input (2 of FIG.3). In this way, the energy is not dissipated in the circuit butreturned to supply and thus there is a resultant increase in efficiency.If, on the other hand, energy is allowed to remain residual in the load8 and output circuitry 12, 7 as freewheeling current, as is the casewith prior art type arrangements, the energy stored may eventually leadto destruction of one or more of the circuit's components. Thisdestruction is obviously a drawback in prior art arrangements.

[0099] Referring to FIG. 4, FIG. 7 (inductive load) and FIG. 8(capacitive load), the input current and voltage can be considered aseither in or out of phase with each other for each cycle, the followingcontrol strategies are adopted to fit each case for each cycle.

[0100] In FIGS. 7 and 8, the switch positions “on’ or ‘off’ areillustrated at the foot of the diagram by way of FIGS. 7A to 7E and 8Ato 8E respectively and refer by arrow to the portion of thecorresponding waveform cycle.

[0101] In FIG. 7, a voltage waveform 34 leading a current waveform 35 isshown.

[0102] In FIG. 8, a current waveform 36 leading a voltage waveform 37 isshown.

[0103] Waveform 38 illustrates control of the MAIN power switch Q1 onand off (at high frequency, in the embodiment shown at 30 KHz) tocontrol the amplitude of the output AC voltage. It should be appreciatedthat the frequency of waveform 38 can be varied to suit the particularapplication of the present invention.

[0104] Waveforms 39 and 40 turn SUB power switch Q3 on and Q2 off (atline frequency in this example) while input voltage is in positive cycle(voltage across points H and C of FIG. 4 is positive). As shown in FIGS.7B and 8B, current flows into load from the active terminal (currentflows from point H to point B of FIG. 4). This will enable the loadcurrent to pass through power switch Q3 and diode D2 while the MAINswitch is off. It should be appreciated that the frequency of waveforms39 and 40 can be varied to suit the particular application of thepresent invention.

[0105] Also waveforms 39 and 40 turn SUB power switch Q2 on and Q3 off(at line frequency, in this example) while input voltage is in negativecycle (voltage across points H and C of FIG. 4 is negative). As shown inFIGS. 7D and 8D current flows into load from neutral terminal (currentflows from point B to point H of FIG. 4). This will enable the loadcurrent to pass through power switch Q2 and diode D3 while the MAINswitch is off.

[0106] The circuit switch configurations are shown in FIGS. 7A, 7C and7E for a leading voltage and 8A, 8C and 8E for a leading current.Enabling MAIN power switch Q1 on and SUB power switch Q2 and Q3 off canalleviate the effects of an input current short circuit and alsoimproves power factor during the periods that the load is in generationstate (which means that during negative cycle the current flows intoactive terminal from load (see FIGS. 7a, 7 e) and during positive cyclethe current flows into neutral terminal from load (see FIG. 7c). ForFIG. 8, the converse applies).

[0107] As can be seen from the description above, Q2 and Q3 have 3different configurations, which are:

[0108] Q2 off, Q3 off, or

[0109] Q2 on, Q3 off, or

[0110] Q2 off, Q3 on.

[0111] Turning to FIG. 4, the control circuit 19 uses sensor 24 (inputvoltage polarity sensing circuit) and sensor 25 (current directionsensing circuit) to sense resistive, capacitive or inductive loads. Thetwo sensors affect the way in which the free wheel subswitches Q2 and Q3are controlled. When the inputs to sensors 24 and 25 are in phase,subswitch Q2 or Q3 is turned on. The main switch Q1 is turned “on/off”with a PWM signal. If the inputs to sensor 24 and sensor 25 are out ofphase, leading or lagging, subswitches Q2 and Q3 are turned off and themainswitch is turned on in accordance with the description above toensure that no short circuit occurs to the input.

[0112] Alternatively, the operation of the subswitches Q2 and Q3 and themain switch Q1 can be controlled in accordance with a comparison of thepolarity of voltages V1 and V2, in a manner in principle similar to thatdescribed above.

[0113] The control of the main switch and the subswitches may behardware and/or software controlled. Hardware is preferred because thereis less component propagation delay between sensing polarity and anappropriate signal being provided to each switch.

[0114] Current through Q1 is monitored in association with resistor R1as shown in FIG. 4. The resistor is in series with the main switch Q1.Voltage is also monitored at R1. By monitoring the resistor R1 insteadof the main switch Q1, batch differences between one main switch andanother main switch from circuit to circuit can be eliminated thus,providing a more reliable input source for the main switch controlcircuit 19 to monitor changes in current and voltage, withoutcompensating for different batch characteristics of Q1 components.

Circuit Design

[0115] A block diagram of control circuit 19 of the AC Power Converterdescribed in this embodiment is shown in FIG. 9. Although control andlogic functions could be handled by the microprocessor 41 in software,it is preferred that over-current/short circuit protection and switchcontrol states are handled by hardware 42 to enable fast controlresponse times. Circuitry associated with the microprocessor 41 is usedto generate a reference PWM signal 43 and to process less time criticalsignals.

Control Signals

[0116] Many different methods and apparatus can be used to generate therequired control signals. Assembly of the apparatus would be within theambit of a skilled person. Various methods of control and logicfunctionality can also be employed utilising the principles of thepresent invention. Nonetheless, there is now describe one example whichis not intended to be construed as limiting the present invention. Inthis example, there are several control signals required to monitor andcontrol, they are:

[0117] PWM

[0118] PWM signal 43 can be in any duty cycle at any moment (includestart up) depending on the required output voltage

[0119] SC STATE (Short Circuit State)

[0120] SC STATE signal 44 will stay high initially when the controlcircuit is powered up. A SC RESET pulse 45 will reset this signal 44. Itwill be set by a MAIN DISABLE pulse 46 or an output short circuitcondition 47.

[0121] SC RESET (Short Circuit Reset)

[0122] SC RESET signal 45 should stay low initially when the controlcircuit is powered up. A pulse of 1 μs or more will enable normal mainswitch operation to occur. This signal preferably occurs at the zerocrossing or while the PWM signal is off.

[0123] MAIN DISABLE

[0124] MAIN DISABLE signal 46 should stay low initially when the controlcircuit is powered up. A pulse of 1 μs or more will disable the mainswitch operation. In FIGS. 7 and 8, a signal of 30 kHz is shown as apreferred signal driving the main switch Q1.

[0125] SUB ENABLE

[0126] SUB ENABLE signal 48 should be low initially when the controlcircuit is powered up and stay high soon after system has powered up. Itshould be high all the time while the main switch Q1 is switching, itmust be high before sending a SC RESET pulse 45. As can be seen fromFIGS. 7 and 8, the frequency of operation of the subswitches Q2, Q3 maybe much reduced compared to that of the main switch Q1. In practice ithas been found, and as is illustrated in FIGS. 7 and 8, that each of thesubswitches Q2, Q3 needs to be enabled only once per cycle. Thus forAustralia, where there is a 50 Hz power cycle, the subswitches Q2, Q3may be switched at 50 Hz, whereas for countries such as USA where thereis a 60 Hz power cycle, the subswitches Q2, Q3 may be switched at 60 Hz.

[0127] LOAD STATE

[0128] LOAD STATE signal 49 will stay high if the load is inductive andstay low if the load is capacitive.

[0129] V-ZERO CROSS

[0130] V-ZERO CROSS signal 50 is a square wave signal at twice linefrequency. The rising edge of a V-ZERO CROSS signal 50 is trigged byeach input voltage zero crossing and its falling edge is trigged by atime delay circuit. On sensing the falling edge of a V-ZERO CROSS signal50, the MICRO 41 reads the value of the output voltage analog signalwhich is used to regulate the output voltage.

[0131] I-ZERO CROSS

[0132] I-ZERO CROSS signal 51 is a square wave signal at twice linefrequency. The rising edge of an I-ZERO CROSS signal 51 is trigged byeach input current zero crossing and its falling edge is trigged by atime delay circuit. On sensing the falling edge of an I-ZERO CROSSsignal 51, the MICRO 41 reads the value of the output current analogsignal which can be used for fault detection in the load.

[0133] OUTPUT VOLTAGE

[0134] OUTPUT VOLTAGE signal 52 is an analog signal which isproportional to the output voltage. The MICRO 41 will use this signal toregulate and display the output voltage value.

[0135] OVER-CURRENT

[0136] OVER-CURRENT signal 53 is an analog signal which is proportionalto the output current. The MICRO 41 can use this signal to display theoutput current value and perform fault detection in the load.

[0137] OVER-CURRENT PRESET

[0138] OVER-CURRENT PRESET signal 54 is an analog signal which isproportional to the current preset value. The MICRO 41 can use thissignal to display the over-load current preset value.

[0139] TEMPERATURE PROB

[0140] TEMPERATURE PROB signal 55 is an analog signal which isproportional to the heatsink temperature. A MICRO 41 will use thissignal to turn the main switch Q1 off when the temperature reaches acertain temperature (say 90° C., but this can be predetermined asdesired). It can be appreciated that some other suitable response canalso be used.

[0141] COMM signal 64 is a communications signal from some other controldevice such as a PC, other microprocessor based system or similar.Typically this signal would be bi-directional and convey informationsuch as required output level, temperature, status etc between the powerconverter and the other system.

[0142] CURRENT DIRECTION signal 65 and INPUT VOLTAGE POLARITY signal 66are used to inform the controller 42 concerning the reactance of theconnected load. Using this information the controller 42 determines theproper sequencing of the main (Q1) and sub (Q2 & Q3) switches in themanner described.

[0143] OUTPUT VOLTAGE LOGICAL signal 67 indicates the state of theoutput voltage. This is a logical signal which, when true, indicatesthat the output voltage is above some preset threshold. This signal isused in conjunction with OVERCURRENT signal 53 by the controller 42 todetermine the presence of a short circuit on the output of the powerconverter. Alternatively, the output voltage signal 67 could be ananalogue signal and the hardware controller 42 could apply the thresholdcriterion.

[0144] MAIN STATE signal 68 is used to override the PWM signal 43 tohold the main-switch Q1 “on” in the case where the CURRENT DIRECTION 65and INPUT VOLTAGE POLARITY 66 indicate that the instantaneous inputvoltage and current to the power converter are out of phase.

[0145] Signals SUB STATE #1 (69) and SUB STATE #2 (70) are signals whichdetermine the “on” or “off” state of the sub switches Q2 and Q3 inaccordance with the principle described and illustrated in FIGS. 7 and8. The hardware controller 42 in accordance with the timing principlesso illustrated determines the required switch states.

Other Aspects

[0146] The filter circuits (L1, L2, C1, C2) as shown on FIGS. 3 and 4are introduced for the purpose of alleviating harmonic effect and tomeet EMI standard. The parameters of the filter circuits are selectedaccording to the main switching frequency which will effect theconverting efficiency.

[0147] A low to medium frequency IGBT or MOSFET can be used for the SUBpower switches Q2, Q3, a high frequency IGBT or MOSFET can be used forthe MAIN power switch Q1.

Transfer Characteristic

[0148] In accordance with another aspect of the present invention, theproblem of charge build up on the output filter capacitor is addressedin a manner that substantially improves the linearity of the circuittransfer characteristic when operating in response to small loadoutputs.

[0149] The output filter is constructed so as to limit the build-up ofcharge within the converter output filter when high impedance loads areconnected. Typically this condition might arise when a Power Converteris used to drive a load substantially less than the converter's rating.Typically the output filter is composed of series inductance andparallel capacitance. In the prior art, the inductance is specified topresent a high impedance to switching harmonics whilst a relatively lowimpedance to line frequency. Conversely, the capacitance is designed topresent low impedance at switching frequencies and high impedance atline frequencies. Together the inductance and capacitance effectivelyblock the passage of switching frequencies and harmonics to the load.Ideally, the performance of the filter is optimal for the highestpossible inductance and capacitance consistent with overall low filterimpedance at line frequency. However, increasing the inductance can giverise to saturation of the inductor core with consequent poor filterperformance.

[0150] In relation to this aspect of the present invention, and withreference to FIG. 4, one embodiment is shown in which the adaptiveinductance is made of two inductor elements, in which a further inductorL3 is added in series with inductor L2. Inductors L2 and L3 can bereferred to as an “adaptive inductance”. This inductor L3 is designed topresent high impedance at switching frequencies and at low currents,which corresponds to low load conditions. In this way the switchingcurrents flowing in the filter are reduced so that the load impedance issufficient to prevent the build-up of charge on the output capacitor.However, when the converter load is increased, the core of the inductorL3 is selected or designed so that it progressively saturates andpresents a relatively low impedance to switching frequencies atrelatively high load conditions. Hence its adaptability.

[0151] For example, the inductance might be constructed using multiplecore elements shared by one or more windings. In this case the corewould be selected, by virtue of material and/or design, so that the netpermeability of the core, and therefore, the inductance possessed acharacteristic generally inversely proportional to current.

[0152] Alternatively, a single core might be employed, that core beingcomposed of material, perhaps composite, such that the requiredpermeability, and thus inductance, characteristic is obtained.

[0153] Essentially, the inductance characteristic should be designed, asnear as possible, so that the charge and discharge time constants, withrespect to switching PWM) frequencies, for the converter output filtercapacitor tend to be equal for all load impedances.

[0154] One example is illustrated in FIG. 12, showing an inductancecomposed of 2 cores (core 72 and core 74) around which a winding 75 isprovided. As noted above, the type of inductor element, core type andcomposition can be varied in order to provide the desired transfercharacteristic.

[0155] The results of this aspect of invention are illustrated in FIG.10. By comparison to FIG. 2 of the prior art, the same transfercharacteristics are denoted the same reference numerals. It isconsidered quite evident that the transfer characteristics illustratedin FIG. 10 are relatively more linear than those illustrated in FIG. 2.The resulting output can also be compared with regard to 50 PWM on eachfigure. In FIG. 2, at this point, the output voltage for line 14 was (anon-linear) 160 volts, rather than 40 volts for line 13. Looking now atFIG. 10, at 50 PWM, line 14 shows a little less than 50 volts, ratherthan 40 volts for line 13. This represents a considerable improvement inthe desired linear relationship for a transfer characteristic betweenPWM and output voltage of a power converter.

[0156] In essence, this aspect requires a saturating inductor L3 inseries with a non-saturating inductor L2, thus by carefully selectingthe value of inductance L3 and its' core material, it is possible tocounteract the charge build-up characteristic in the output filtertypical with the prior art. This will obviously vary dependent upon heapplication and desired characteristic sought for the power converter.In the present embodiment, inductor L2 is a bobbin type with a MnZncore, and inductor L3 is a toroidal type with a MnZn core. The presentinvention is not to be limited to the use of only these types ofinductors. The present invention may be used in a switch mode powerconverter or an AC power converter.

Surge Detection

[0157] The present invention also serves to detect surge currents. If acurrent spike is detected, the pulse width is decreased (PWM turned OFF)and accordingly, the amplitude of the spike is reduced thus, overcomingor controlling the output of the spike.

[0158] In another aspect of invention there is provided a method ofcontrolling over voltage or over current conditions in a circuit bysampling the over-voltage or over-current at a frequency which is highenough to attenuate the amplitude of the signal output to the circuit.

[0159] Referring to FIG. 4, consider a current change through outputfilter inductors L2 and L3 as:

ΔI=V2/L*ΔT

[0160] here

[0161] ΔT is the time period during which the main switch Q1 is on

[0162] ΔI is current change during the time period ΔT

[0163] V2 is the voltage which crosses the inductor L2 during the timeperiod ΔT

[0164] L is the inductance of inductors L2 and L3

[0165] By controlling the main switch Q1 on time, ΔT effects the currentchange ΔI. When an overload occurs, the voltage V2 will increase, whichwill in turn increase the current ΔI during the same period ΔT. Bycomparing the instantaneous current with a preset current level (53 and54 respectively of FIG. 9) during each switching cycle, if the transientcurrent is over the preset current level, main switch Q1 can be turnedoff earlier than it would have been. This will limit the average currentthrough inductor L2 and in turn will limit the maximum load current.

[0166] Changing the preset current level 54 will change the turn-on timeof main switch Q1 in affect setting the maximum load current. Each timea transient current is detected over the preset current level, mainswitch Q1 is turned off which in turn will effect the maximum loadcurrent.

[0167] By sensing output voltage and transient currents it is possibleto discriminate between an overload and a short circuit outputcondition. If the output voltage is within a certain level whileover-current occurs, an overload condition is detected. If outputvoltage is near zero while over-current occurs, a short circuitcondition is detected. If a short circuit condition is detected, themain switch Q1 is turned off permanently.

Networking

[0168] A further aspect of the present invention relates to problemsencountered when a number of power converters are used collectively tocontrol a number of associated loads. Referring to FIG. 11, one load 56,comprising one or a number of lights or other types of loads isconnected to a single power converter, called a “Channel”. A number ofchannels can be provided in a modular form as a unit 58, in which casesome of the control logic of each converter (see feature 19 of FIG. 3)is shared by a number of channels. A number of units may be housedtogether in a rack 59. Equally, a unit may be free standing.

[0169] The units can be interconnected for the purpose of remote controland monitoring by a network 71. A variety of network types may be usedsuch as DMX, SMX, ACN, AMX etc. These networks allow remote control ofthe output level of each channel within a unit. Usually the processingmeans for facilitating the network communications is provided on a Unit58 or rack 59 basis. Regardless of the configuration, each of thechannels in the system will be uniquely addressable.

[0170] In prior art arrangements, each unit is provided with a userinterface control panel via which the operator is able to access thesetup of each unit and/or channel contained within that unit, and toobserve various parameters associated with the unit and its channels.Each unit would have its own corresponding control panel that couldaccess the function of that unit alone.

[0171] In this aspect of the invention, a Control Network 60 is providedconnecting a plurality of units through a network such as Ethernet orLON™. In this aspect of the invention, the unit control panel 62 isconnected to the unit via the Control Network 60. By this means eachcontrol panel 62 can be arranged via appropriate logic and/or softwareto access the functions of every unit connected to the control network60. In this way the human interface required to give instruction ormonitor performance, when necessary, may be provided through thenetwork.

[0172] In the current aspect, the Control Network 60 comprises aphysically separate network to the Lighting Control network 71 howeverit is possible to combine these two logical networks onto one physicalnetwork.

[0173] The human interface may be provided through a familiar personalcompute (PC) 61 or similar generic controller, or a compatible lightingcontroller 63, or a dedicated control unit 62. It is preferred forsimplicity and convenience, to have a dedicated control unit capable ofbeing used either locally or remotely. When used locally this controllermay be mounted onto the power control module (unit) 58 as an integralpart of the unit. This both complements the design aesthetics andprovides a robust and reliable human interface. When used remotely thecontrol unit 62 may be operated as a stand alone controller mounted on awall, hand held, or other such convenient location. The controller hasaccess to all channels, units or racks connected to the network.

[0174] This means that, say, one user interface module may be used tocontrol a multiplicity of dimmer units or racks. This local controlmodule may be located either nearby or remotely. This remote controlnetwork may be independent from the lighting control network.

[0175] It also means that the user interface may be removed entirely(since it is not essential for the basic function of the dimmer pack)thus adding security from tampering.

[0176] The essential the aspect of this invention is that the now commonuser interface of the dimmer itself can be shared among a number ofunits because of the network-able aspect of it.

[0177] In the present invention, each unit 58 may be optionally fittedwith an additional user interface panel 72 providing a minimum subset ofthe total unit functions so that the unit can function without a controlpanel 62 connected at all.

The claims defining the invention are as follows:
 1. A power converterincluding: input means for receiving supply power, switch meansresponsive to control means for providing control of power delivered toan output load, and detecting means to detect a difference in polarityor amplitude between selected waveforms or points in the converter, inwhich the control means controls the operation of the switch means in amanner that enables the switch means to be ‘on’ when required forcontrol purposes.
 2. A power converter as claimed in claim 1, whereinthe switch means is enabled ‘on’ when there is a difference in polaritydetected.
 3. A power converter as claimed in claim 1 or 2, in which theswitch means is enabled ‘on’ when a difference in polarity betweenvoltage and current waveforms is detected.
 4. A power converter asclaimed in claim 3, in which the current waveform is input current andthe voltage waveform is input voltage.
 5. A power converter as claimedin claim 1, in which the switch means is enabled ‘on’ when a differencein polarity between input voltage and voltage across the switch means isdetected.
 6. A power converter as claimed in any one of claims 1 to 5,in which the power converter is a high frequency switch mode converter.7. A dimmer including the power converter of any one of claims 1 to 6.8. A method of reducing energy dissipation in a power converter, themethod including the steps of: detecting a polarity difference betweenselected waveforms or points in the converter, enabling a switch ‘on’ inresponse to detecting opposing polarity.
 9. A method as claimed in claim8, in which the switch is enabled ‘on’ when a difference in polaritybetween voltage and current waveforms is detected.
 10. A method asclaimed in claim 9, in which the voltage waveform is input voltage andthe current waveform is input current.
 11. A method as claimed in claim8, in which the switch is enabled ‘on’ when a difference in polaritybetween input voltage and voltage across the switch is detected.
 12. Apower converter incorporating the method of any one of claims 8 to 11.13. A method of controlling over-current conditions in a circuit,including the steps of: providing PWM sampling of the input waveform,sampling the input waveform, measuring current as it passes through amainswitch to detect an over-voltage or over-current condition, turningthe mainswitch off in response to the detection of the over-voltage orover-current condition, and adjusting the PWM sampling in order toattenuate voltage or current through the converter.
 14. A method asclaimed in claim 13, further including sensing output voltage andtransient currents so as to discriminate between an overload and a shortcircuit output condition.
 15. A method as claimed in claim 14, in which:if the output voltage is within a predetermined level while theover-current condition occurs, an overload condition is detected,whereas if the output voltage is substantially zero volts while theover-current condition occurs, a short circuit condition is detected.16. A control unit adapted to control a power converter, the controlunit being provided integrally with the power converter in which thecontrol unit is attached to and communicates directly to the powerconverter, or remotely of the power converter in which the control unitis detached and communicates remotely via a suitable mode ofcommunication to the power converter.
 17. A network of power convertersbeing at least two power converters coupled via a communication network,and including the control unit of claim
 16. 18. A network as claimed inclaim 17, in which the control unit is also operable to control at leastany selected one of the power converters.
 19. A network as claimed inclaim 17 or 18, in which messages and other required information iscommunicated between the power converters via a control unit.
 20. An LCfilter including: a capacitive element, a first inductance having arelatively high impedance at switching frequency and a relatively lowimpedance at line frequency, the relative impedance of the firstinductance being relative to the load impedance, the improvementincluding: a second inductance having a relatively high impedance atswitching frequency and at line frequency when current is relativelylow, and the second inductance having a relatively low impedance atswitching frequency and at line frequency when the current is relativelyhigh.
 21. An LC filter in which the first and second inductances areprovided in one inductive element.
 22. An AC power converter includingthe LC filter of claims 20 or
 21. 23. A power converter as claimed inclaim 1, including the adaptive inductance of claim 20.